1. Field of the Invention
The present invention relates to an optical recording medium and an optical read/write apparatus and, more particularly, to an improvement in the recording density of the optical recording medium.
2. Description of the Prior Art
Optical recording technology has been widely used in the high density recording of digital information. Representative of this technology is the compact disc (trademark of Philips and Sony) used for an optical recording medium. The compact disk is now being used as a read only memory for a computer. The optical compact disk is protected by a transparent protective layer having a thickness of 1.2 mm. Further, the optical compact disc halving densely recorded information uses a powerful error correction code and accordingly is compatible with any other medium.
The International Standard 9171 has specified the optical disk for use in an external memory of a computer. There are two types of the Standards of A format and B format. An optical disk of either type can read and write information through the transparent protective layer of 1.2 mm thickness with an objective lens in a numerical aperture (NA) of 0.55. A process of reading and writing information is executed through the transparent protective layer which acts as a protective layer. A laser beam for reading and writing information is focused through the transparent layer of 1.2 mm thickness. Since the laser beam is well defocused on the surface of the transparent layer, dust adhered to the surface does not result in signal-drop-out which causes an inaccurate reading of a disk so long as the dust is not large. The dust size is surely lower than 0.2 mm under the usual environment. The laser is focused with a objective lens in a numerical aperture of 0.55 through the transparent layer having a refractive index of 1.5 and forms a laser beam in a diameter of about 1 mm at the surface of the optical recording medium. The dust having a size lower than 0.2 mm does not prevent the laser beam from focusing and reading correctly. Further, the optical disk employs an error correction code named as a reed-solomon code (RS code). Either of the A format and B format uses the RS code generated at the Galois Field-GF (2.sup.8). One byte is made one symbol which is one unit of the RS code. The A format has 520 bytes (520 symbols) arranged at 104 rows and 5 columns at two dimensions. The 104 symbols in a column direction are added with parities of 16 symbols and form the RS code (120,104). Such a long RS code is named as a Long Distance Code (LDC). An information added with the parity can be read sequentially in a row direction from the first row to the 120th row and recorded sequentially on the recording medium. Each symbol in a column direction of the RS code can be arranged at every interval of five symbols on the recording medium. A process to arrange each symbol of a code word at intervals on the recording medium is named as a interleave. The A format mentioned above has an interleave length of five. On the other hand, the B format has an information of 528 bytes arranged at 44 rows and at 12 columns in two dimensions. The RS code (14,12) can be formed by 12 symbols in a row direction added with 2 symbols of parity. The RS code (48,44) can be set up by 44 symbols in a column direction added with 4 symbols of parity. A product code (PC) having a parity added with a row direction and column direction has a correcting power lower than that of the LDC but can obtain a high correcting power by executing an alternate correction at the row direction and the column direction. The detailed explanation on the data correction operation is disclosed in "A VLSI Design of a Pipeline Reed-Solomon Decoder" IEEE Transactions on Computers, vol. c-34, No. 5 by H. M. Shao and others.
In connection with an optical disk, a laser having a wave length of L forms a spot diameter of L/NA. The spot diameter limits the recording density. A objective lens having a higher numerical aperture results in a higher recording density. However, an objective lens having a higher numerical aperture can not always make a laser beam substantially smaller because the laser beam focused through a transparent layer 1.2 mm thickness easily generates aberration for the inclined recording disk. A transparent layer in a lower thickness can make smaller the aberration due to the inclined disk. The transparent protective layer in a thickness lower than 1 mm results in an appreciable degradation of a signal due to the dust adhered to the surface of the transparent protective layer. There are so many errors that the conventional error correction code can not correct the errors. Especially, the B format having only 4 symbols of parity added may erroneously correct a code word including errors which can not essentially be corrected with another code word when the transparent protective layer having a lower thickness generates frequent errors.
The erroneous correction is a problem which can not be neglected for high density recording.